1. Field
Example embodiments generally relate to an image forming apparatus, such as a copying machine, a facsimile machine, and a printer, a process cartridge employed in the image forming apparatus and a cleaning device employed in the image forming apparatus or the process cartridge.
2. Description of the Related Art
A related-art image forming apparatus, such as a copying machine, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image bearing member (e.g., a photoconductor). An optical device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data. The electrostatic latent image is developed with a developer (e.g., a toner) to form a toner image on the photoconductor. A transfer device transfers the toner image formed on the photoconductor onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
The related-art image forming apparatus further includes a cleaning device including a cleaning blade. The cleaning blade includes an elastic member, and contacts the surface of the image bearing member to remove residual toner particles from the surface of the image bearing member. Such a cleaning method is known as a blade cleaning method, and is widely used by virtue of providing stable cleaning performance with a simple configuration.
To meet a demand for higher quality images, toner particles having a smaller particle diameter and a spherical shape are developed in recent years. The toner particles having a smaller particle diameter provide images with higher accuracy, definition, and resolution. The toner particles having a spherical shape improve development and transfer performance.
However, it is difficult to remove the toner particles having a smaller particle diameter and a spherical shape from the surface of the image bearing member by using the blade cleaning method due to a tiny space formed between the image bearing member and the cleaning blade. When the cleaning blade contacts the surface of the image bearing member to remove the toner particles from the surface of the image bearing member, an edge portion of the cleaning blade may be deformed due to friction resistance with the surface of the image bearing member. As a result, a stick-slip motion occurs, causing the tiny space between the image bearing member and the cleaning blade. The smaller the toner particles are, the easier the toner particles are to enter the tiny space. Moreover, the rounder the toner particles entering the tiny space are, the easier the toner particles are to roll in the tiny space due to rotational moment. As a result, the cleaning blade is pushed up by the toner particles, so that the toner particles easily enter the tiny space between the image bearing member and the cleaning blade. Therefore, the cleaning blade cannot remove the toner particles from the surface of the image bearing member.
One possible technique of preventing the toner particles from entering the tiny space is to increase linear pressure of the cleaning blade against the image bearing member. However, high linear pressure causes high loads on the image bearing member and the cleaning blade. As a result, the image bearing member and the cleaning blade are worn away, shortening a product life.
One example of a cleaning device uses an electrostatic cleaning method in order to remove the toner particles having a smaller particle diameter and a spherical shape from the surface of the image bearing member. A voltage with the polarity opposite to that of the toner particles is applied to an electrostatic cleaning member such as a conductive cleaning brush in contact with the surface of the image bearing member, so that the toner particles are electrostatically removed from the surface of the image bearing member. However, the toner particles may not be removed from the surface of the image bearing member even by using the electrostatical cleaning method due to a variation in charge amount of the toner particles conveyed to the cleaning device. For example, FIG. 3 is a graph illustrating charge distributions of the toner particles on the surface of the image bearing member before and after transfer is performed at normal temperature and humidity. As shown in FIG. 3, most of the toner particles on the image bearing member before transfer is performed are charged to the negative polarity, which is a regular polarity of the toner particles. In the transfer device, a transfer bias with the polarity opposite to that of the toner particles, namely, the positive transfer bias, is applied to the toner particles on the surface of the image bearing member, so that the toner particles thereon are transferred onto a transfer sheet. However, because the polarity of a part of the toner particles on the surface of the image bearing member may be reversed to positive due to the positive charge injected from the transfer device, such toner particles may remain on the surface of the image bearing member after transfer has been performed, resulting in residual toner particles. Therefore, the residual toner particles on the image bearing member after transfer has been performed has a broader charge distribution including both the positively charged toner particles and the negatively charged toner particles as shown in FIG. 3. In the electrostatic cleaning method described above, a positive voltage, which is opposite to the polarity of the toner particles, is applied to the cleaning brush to electrostatically remove the toner particles from the surface of the image bearing member. Therefore, it is difficult to remove the toner particles with the polarity reversed to positive by using the positively charged cleaning brush.
Another example of a cleaning device is proposed in which a polarity control unit to control the polarity of the residual toner particles is provided on an upstream side of the electrostatic cleaning member. The polarity control unit controls the residual toner particles on the surface of the image bearing member to have the negative polarity, which is a regular polarity of the toner particles, so that the positively charged cleaning brush provided on a downstream side of the polarity control unit can easily collect the toner particles.
Such a polarity control units uses a micro discharge of a corona charger provided apart from the surface of the image bearing member, and a charge injection from an energized conductive brush roller in contact with the surface of the image bearing member. A compact polarity control unit with a simple configuration, which uses a charge injection from an energized conductive blade, is also proposed.
However, the polarity control units described above simultaneously charge the image bearing member bearing the residual toner particles thereon when controlling the polarity of the residual toner particles. Consequently, the surface of the image bearing member which is charged to a highly negative potential is conveyed to the cleaning brush to which the positive voltage is applied. Because the cleaning brush includes a brush string including a conductive material, a positive charge may be injected into the residual toner particles between the surface of the image bearing member and the cleaning brush. Particularly, when a potential gradient between the surface of the image bearing member and the cleaning brush is large, a larger amount of current flows into the residual toner particles between the surface of the image bearing member and the cleaning brush in order to compensate the potential gradient, and the positive charge is injected into the residual toner particles. Thus, the polarity of the residual toner particles is reversed to positive, so that the cleaning brush may not collect the residual toner particles with the positive polarity. As a result, a larger number of cleaning residual toner particles with the positive polarity remains on the surface of the image bearing member.